The present invention relates to the recovery of uranium from subterranean ore deposits and more particularly to an in-situ leaching operation employing a near neutral or alkaline lixiviant in which the predominant leaching agent is an alkali metal sulfate.
In an in-situ leaching operation, a lixiviant is introduced into a subterranean uranium ore deposit through a suitable injection system. The lixiviant may be an acidic or alkaline medium which solubilizes uranium values as it traverses the ore body. The pregnant lixiviant is then withdrawn from the ore body through a production system and treated to recover uranium therefrom by suitable techniques such as solvent extraction, direct precipitation or by adsorption and elution employing an ion exchange resin.
In acid leaching operations, the most commonly employed acid is sulfuric acid. The sulfuric acid normally is present in the lixiviant in a concentration to provide a pH of 2 or less. Normally, sufficient acid is present in the injected lixiviant to provide an excess of acid over that consumed by uranium solubilization and inorganic carbonates within the formation in order to retain a relatively low pH in the pregnant lixiviant as it is withdrawn from the subterranean deposit. For example, as disclosed in Merritt, R. C., THE EXTRACTIVE METALLURGY OF URANIUM, Colorado School of Mines, Research Institute, USA (1971) at page 63, it is desirable to provide excess acid in order to prevent reprecipitation of uranium. Thus, Merritt discloses that uranium may precipitate from solution in the presence of various anions in sulfate solutions at pH's within the range of 1.3-6.0 depending upon temperature and the concentrations of various constituents in solution. For example, Merritt in Table 5-2 discloses that uranyl ions will precipitate from sulfate solutions in the presence of carbonate ions if the pH of the lixiviant is allowed to increase to a value within a range of 3.5-6.0.
The presence of carbonate materials in subterranean rock deposits containing uranium limits the use of acid lixiviants not only with respect to acid consumption by the carbonates but also due to the precipitation of reaction products such as calcium sulfate which may result in plugging of the formation. Thus, the use of an alkaline lixiviant is strongly indicated in many in-situ leaching operations, not only because of the carbonate content of the rock, but also since the alkaline lixiviants are more selective with respect to uranium dissolution than are the acid lixiviants. Alkaline lixiviants normally employ carbonate ions, added as alkali metal carbonates or bicarbonates or mixtures thereof to complex the uranium in the form of the watersoluble uranyl tricarbonate ion. Thus, U.S. Pat. No. 2,896,930 to Menke discloses in-situ leaching employing a "cold" aqueous solution of an alkali metal carbonate, e.g. sodium or potassium carbonate or bicarbonate, in a concentration of less than 50 grams per liter. Menke discloses that in order to increase the solubility of uranium in the cold leach solution it is useful to incorporate complexing agents capable of forming little-ionized complexes with uranium-bearing ions. The patentee lists a large number of such complexing agents including those which yield sulfate ions.
In many cases, the uranium in the subterranean deposit exists in the tetravalent state. Thus, it is a conventional practice in both acid and alkaline leaching to employ an oxidizing agent to ensure that the uranium is oxidized to or retained in the hexavalent state at which it is solubilized by the lixiviant. In in-situ leaching operations employing an alkaline lixiviant, the most commonly employed oxidizing agents are hydrogen peroxide as disclosed in the aforementioned patent to Menke or air as disclosed in U.S. Pat. No. 2,954,218 to Dew et al. An especially suitable oxidizing agent for use in conjunction with alkaline lixiviants in leaching refractory ores is an alkali metal or alkaline earth metal hypochlorite as disclosed in copending application Ser. No. 928,676 entitled PROCESS FOR THE IN-SITU LEACHING OF URANIUM, filed July 28, 1978 by Edward Thomas Habib, Jr. and Thomas C. Vogt, Jr.